F2R and chronic obstructive pulmonary disease: In studies like the one reported here, where the frequency of the rs2227744A allele in infrequent exacerbators was found to be similar to that observed in the general population (Whitehall II and ELSA healthy controls), and the effect of the SNP on a subgroup of cases was diluted and masked, a comparison between all COPD cases (including both frequent and infrequent exacerbators) and healthy controls would potentially fail to show an association between PAR-1 and susceptibility to COPD.